Prevent updating CNF and CR sdewanpuporse label

Prevent updating CNF and CR sdewanpuporse label

In sdewan, we use label 'sdewanpurpose' to identify a cnf and to match
with CRs. Updating cnf sdewanpurpose label value means that deleting old
cnf and creating a new cnf. But K8s can only receive an "UPDATE" event,
reconcile can only get the current info of the CNF, no previous label
value. So it can't remove the old rules.
This patch is to prevent updating CNF and CR sdewanpurpose label for
simplify.

Signed-off-by: chengli3 <cheng1.li@intel.com>
Change-Id: I75b7d400981f3103b02c9d73f68d8b62db7da899